1. Field of the Invention
The present invention relates to a scanning exposure device and, more particularly, to a scanning exposure device provided with a moving mirror which deflects a laser beam falling thereon.
2. Description of Related Art
A scanning exposure device is provided with a moving mirror that deflects a laser beam emitted by a laser light source for scanning the laser beam on a photoconductive drum. Such a scanning exposure device is, for example, disclosed in U.S. Pat. No. 4,847,492 and U.S. Pat. No. 4,926,263 and the scanning exposure device is usually applied to printers and copying machines. A rotating polygon mirror is used in the scanning exposure device as the moving mirror which deflects the laser beam falling thereon. The scanning exposure device includes a scanning exposure control unit for controlling the laser light source to modulate the laser beam.
When the scanning exposure device is applied to formation of a latent image corresponding to a white image on the photoconductive drum, the scanning exposure control unit controls the laser light source so that the intensity of the laser beam is fixed. When the scanning exposure device is applied to formation of a latent image corresponding to a black image on the photoconductive drum, the scanning exposure control unit controls the laser light source so that the laser beam does not fall on the photoconductive drum. In forming a latent image corresponding to a gradated image on the photoconductive drum, the scanning exposure control unit controls the laser light source so that the intensity of the laser beam emitted by the laser light source is determined in order to obtain desired intensity of the laser beam on the photoconductive drum.
However, such a manner of controlling the intensity of the laser beam emitted by the laser light source causes incorrect exposure on the photoconductive drum when a variation in reflectance of the mirror occurs. The reason is that variation in reflectance of the moving mirror affects the intensity of the laser beam reflected by the moving mirror.
The reflectance of the moving mirror varies with the angle of incidence of the laser beam on the moving mirror. The angle of incidence of the laser beam on the moving mirror varies with the angular position of the moving mirror during rotation.
The variation in the intensity of the reflected laser beam is dependent on the angular position of the moving mirror as will be explained in more detail with reference to FIG. 8. A laser beam emitted by a laser light source 10 falls on a mirror 12. As the mirror 12 is turned in the direction of an arrow C from an angular position A, indicated by a continuous line, to an angular position B, indicated by an alternate long and short dash line, the laser beam is deflected by the mirror 12 as indicated by an arrow D to sweep the surface of a charged photoconductive drum 14 for scanning exposure from the left end 14a to the right end 14b. As the mirror 12 is turned from the angular position A to the angular position B, the angle of incidence of the laser beam decreases from an angle .theta.1 to an angle .theta.2, and the reflectance of the mirror 12 decreases accordingly.
Consequently, the intensity I of the reflected laser beam falling on the photoconductive drum 14 decreases as the spot of the laser beam moves from the left end position 14a to the right end position 14b even if the laser light source 10 emits a laser beam of a fixed intensity and, consequently, the local charge of the photoconductive drum 14 is greater for positions nearer to the right end position 14b, as the discharge is reduced as a result of the reduced reflected light, and hence the density of the printed image varies accordingly.
The incorrect exposure is not a very significant problem in forming a gradated image in the low gradations used by conventional printers or copying machines. However, in forming a gradated image on a recording sheet, for example, having a fine gradation of 256 steps of density (division of densities between white and black into 256 levels of density), the intensity of the laser beam must be controlled in 256 intensity levels. Therefore, the variation of the intensity of the reflected laser beam due to the variation of the angular position of the moving mirror affects the gradation of the recorded image significantly.